1. Field of the Invention
The present invention generally relates to an acceleration sensor (accelerometer) for sensing the change of velocity with respect to time. More specifically, the present invention is directed to a semiconductor accelerometer of the type using a strain-gauge, utilizing the change in resistance caused by expansion or constriction of a semiconductor, and also to a method for testing the acceleration sensor.
2. Description of the Prior Art
Heretofore, accelerometers have been used in many industrial fields and are generally grouped into the following types:
(i) a servo type accelerometer that detects a displacement of a weight member or a thin plate from a balanced position when it receives the influence of acceleration; PA1 (ii) a piezoelectric type accelerometer that uses a piezoelectric material which is responsible for generating electric force or charges in relation to a degree of strain that is caused; and PA1 (iii) a strain-gauge type accelerometer that uses the variation in resistance caused by the expansion and contraction of a metal or semiconductor. PA1 In a first aspect of the present invention, there is provided a semiconductor acceleration sensor, comprising: PA1 a silicon detecting member having a weight, a supporting frame, and a beam for coupling the weight with the supporting frame, which are integrally processed from a silicon wafer; PA1 at least one semiconductor strain gauge formed on a surface of the beam; PA1 a glass substrate electrostatically joined with the supporting frame of the silicon detecting member; and PA1 a gap portion formed between a surface of the glass substrate and a lower surface of the weight. PA1 (i) providing a semiconductor acceleration sensor as a test device, having: a silicon detecting member having a weight, a supporting frame, and a beam for coupling the weight with the supporting frame, which are integrally processed from a silicon wafer; at least one semiconductor strain gauge formed on an upper surface of the beam; a glass substrate electrostatically jointed with the supporting frame of the silicon detecting member; and a gap portion formed between an upper surface of the glass substrate and a lower surface of the weight, PA1 (ii) applying an alternating current on the conductive film so as to generate a potential difference between the silicon detecting member and the conductive film, and PA1 (iii) generating false acceleration by an effect of electrostatic force between the conductive film and the lower surface of the weight to perform a calibration. PA1 the silicon detecting member has a through-hole or recess for permitting external wiring to be connected to the conductive film, formed at a portion facing the wiring part of the glass substrate. PA1 preparing a silicon detecting member having a weight, a supporting frame, and a beam for coupling the weight with the supporting frame, which are integrally processed from a silicon wafer, where at least one semiconductor strain gauge is formed on an upper surface of the beam; PA1 forming a conductive film and a wiring portion thereof on a surface of a glass substrate; PA1 electrostatically jointing the glass substrate with the supporting frame of the silicon detecting member; and PA1 forming a gap portion between an upper surface of the glass substrate and a lower surface of the weight. PA1 the silicon detecting member has a through-hole or recess for permitting external wiring to be connected to the conductive film, formed at a portion facing the wiring part of the glass substrate.
In the automobile industry, by way of example, accelerometers have been used in so-called airbag systems for ensuring the safety of a passenger on the occasion of a traffic accident. The airbag system is responsible for preventing the driver from bumping against the steering wheel or the like by a sudden expansion of the airbag when an accelerometer therein senses a sudden reduction in velocity as a result of an accident or the like. In this case, the driver's safety is directly depended on the accelerometer's condition, so that the accelerometer of the airbag system must be of high reliability. Therefore, a small-sized semiconductor accelerometer comprising strain-gauges on a semiconductor substrate has been tried out in the above airbag system.
In general, there are two types of semiconductor strain-gauges, a bulk type and a diffusion type. The conventional methods for manufacturing integrated circuits can be used for preparing the diffusion-type strain-gauge, so that it is possible to integrate an amplification circuit, a compensation circuit, and the like on a common substrate. Furthermore, the influence of temperature variation on the strain-gauge can be precisely compensated by integrating a bridge circuit into the strain-gauge.
Usually, the performance of a semiconductor accelerometer can be estimated by performing a vibration test (accelerating test) using a large-sized vibration test machine. For manufacturing an accelerometer that works stably at all times, the accelerometer is subjected to processing which includes the step of compensating for variations in the accelerometer's sensitivity by using the compensation circuit.
In order to perform the above test in large quantities, a plurality of the test machines should be operated simultaneously for testing a plurality of the semiconductor accelerometers at the same time. Thus, the process takes a lot of time and leads to extremely high production costs.
To solve the problems, as shown in FIG. 1, Allen and his coworkers have proposed an accelerometer which is capable of testing and calibrating itself in U.S. Pat. No. 5,103,667.
In FIG. 1, the accelerometer comprises a silicon frame 120, a silicon cap 140, a silicon mass 110, and a silicon base 150. In addition, the silicon mass 110 is supported by the silicon frame 120 with the aid of flexures 112, 114 on which piezo-resistors 130, 132 are formed. An opening or recess of the cap 140 is oriented toward the silicon frame 120 so as to form an air gap 142 between these members. As shown in the figure, furthermore, there is a deflection electrode 160 disposed on an inner surface of the cap 140. On the other hand, the mass 110 is arranged so as to look toward the silicon base 150 and form an air gap 152 between these members. The frame 120 has a pad 161 on its surface. The electrode 160 is connected with the pad 161 electrically by means of a metal conductor 180 formed on a surface of the cap 140.
In the conventional example of FIG. 1, the silicon frame 120, the silicon cap 140, and the silicon base 150 are connected or bonded together. If a bonding agent, solder, or other possible means for connecting them together is used to realize the configuration in FIG. 1, there is a possibility of deterioration of the bonding or connecting layers. Therefore, there is a danger that the reliability of the accelerometer may fall. In order to control the air gaps 142 and 152, the thickness of the connecting or bonding layers should be also controlled. However, this is technically hard for one to do.
By the way, electrostatic-bonding (anode-bonding) has been known as a bonding process without using any bonding agent or bonding material. The process includes the steps of contacting the silicon and the glass and applying a voltage of about 600 volts to them at 300-500.degree. C. to cause migration of alkali-ions of the glass toward their interface, resulting in a space-charge layer forming around their interface. The space-charge layer is responsible for generating static electricity to form a chemical bond between the silicon and the glass. The chemical bond may be a result of forming SiO.sub.2 in the interface between the silicon and the glass by binding Si with O.sup.- that migrates through the glass under the electric field.
In the case of using the electrostatic-bonding process for bonding two silicon materials together, as shown in the U.S. patent document described above, a SiO.sub.2 membrane is formed on a bind surface of each silicon material to be attached by means of wet-oxidation, and at the same time a lot of SiOH groups are formed in the SiO2 membrane because of using H.sup.+ as a carrier for the reaction represented as: EQU SiOH.fwdarw.SiO.sup.- +H.sup.+
However, an aluminum material, which is generally provided for electrical wiring, cannot be used in the above process of electrostatic-bonding because the process includes the step of subjecting the material to the high temperature of 900.degree. C. During the process, furthermore, the device on which the circuit is formed can be affected by such a thermal condition. Consequently, it is difficult to use electrostatic-bonding in the process of manufacturing semiconductor accelerometers in a practical manner.
To solve this problem, one can consider using a glass cap instead of the silicon cap 140 in order to perform the step of electrostatic bonding with the silicon frame at the temperature of 300 to 500.degree. C.
FIGS. 2A and 2B show one example of a conventional accelerometer in which a cap layer 200 and a substrate 300 are made of glass while a detector 100 is made of silicon. They are joined together as layers by means of electrostatic-bonding at a relatively low temperature. In these figures, FIG. 2A is a cross-sectional overall view of the conventional accelerometer and FIG. 2B is an enlarged view of the part surrounded by a broken line in FIG. 2A.
An inner side hollow 201 of the cap 200 has an electro-conductive membrane 202 which extends toward a bonding surface 400 thereof and provides electrical wiring connected with the silicon detector 100. A plurality of semiconductor strain-gauges 104 on flexures 101 form a Wheatstone bridge circuit having a ground that is connected to both the silicon detector 100 and the mass 102. The silicon detector 100 comprises a frame 103 for supporting the mass 102 by the flexures 101. A surface of the frame 103 is electrostatically connected with the base 300 so as to form a hollow 301.
For realizing the above construction, however, it is necessary to connect the electrode 202 with the silicon detector 100 by passing it from the cap's hollow 201 to the bonding or contacting surface 400. As shown in FIG. 2B, furthermore, the cap's end facing to the base should be formed as an inclined plane to make a space between the cap and the base, so that it is difficult to keep the bonding or connecting surface 400 even. Therefore the bonding or connecting surface 400 would not be reliable with respect to its mechanical strength. Especially in the case of using the accelerometer as a collision-detecting sensor in an automobile's airbag system, the sensor should be constructed to have high reliability in its mechanical strength. Otherwise, the sensor would be inferior and may cause a fatality. For avoiding such problems, one might consider enlarging the contacting and bonding areas. However, this would cause other problems such as an increase of its manufacturing cost as a result of increased chip-size. In general, glass material can be subjected to the process of electrostatic bonding. Comparing glass with silicon, however, it is very difficult to make a small recess (5-15 .mu.m) on a surface of the glass and it costs a great deal.
Furthermore, a chip can in general be sliced into wafers having a three layer structure of different materials (i.e., silicon and glass) by using a dicing cutter. In this case, however, the slicing must be performed at an extremely low speed of 1-5 mm/second because of slicing the chip made of different materials, resulting in high manufacturing cost.